A conventional system is known which conducts redundant transmission by repeating the same information for a plural number of times so as to minimize the number of bit errors on a transmission line. According to this system, as the number of repeating increases, the redundancy will increase to improve the accuracy of information which is to be transmitted. However, since the mark rate of a series of information is not necessarily 50%, a DC component will be generated in the signal which presents difficulties over transmission lines and circuits of the DC cut type. Moreover, if marks or spaces are continuously included in a code to be transmitted, it becomes difficult to extract clock signals.
A split phase code is also known, as a system which repeatedly transmits identical codes continuously in positive and negative polarities. The code is shown in FIG. 1 wherein the letter M stands for a mark of one bit information and the letter S stands for a space. In this code system, as the polarity always inverts within one bit information, no DC component will occur in the signal. Further, as the signal level repeatedly crosses the zero level, clock signals can always be extracted. However, when a split phase code is transmitted to a line having pulsating noise or short cycle fading, the error rate is not necessarily improved since repeating information follows, one after another, closely timewise and correlation with the same noise or fading becomes large.
In the split phase code, as the polarity inverts at the center of a bit, the eye pattern thereof will have wide open eye apertures 21 and small open eye apertures 22 as shown in FIG. 2. If a transmitted information signal is random, the probability that it will pass through these two eye apertures 21 and 22 is 1/2 respectively. When a code passes through a small open eye aperture 22 at the probability of 1/2, it is likely to be subjected to inter-code interference.